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Abstract:

A wireless communication system is disclosed. The wireless communication
system performs data transmission using spatially multiplexed streams
from a first terminal including N antennas to a second terminal including
M antennas (N and M are integers larger than or equal to 2 and N>M).

Claims:

1-14. (canceled)

15. A wireless communication apparatus including a first number of
antennas for receiving spatially multiplexed streams from an initiator
including a second number of antennas, the second number being an integer
equal to or greater than two and the first number being an integer equal
to or greater than one, the apparatus comprising: notification receiving
means for receiving a maximum dimension when computing a transmission
weight matrix for beamforming of the initiator, the maximum dimension
being an integer equal to or less than the second number; channel matrix
estimating means for receiving a packet including a training sequence
corresponding to the second number of antennas of the initiator and the
first number of antennas of the wireless communication apparatus from the
initiator, and estimating a channel matrix having the first number of
rows and the second number of columns; channel information feedback means
for suppressing the channel matrix estimated by the channel matrix
estimating means to a third number of rows and the second number of
columns, the third number being equal to or less than the maximum
dimension, and feeding back the suppressed channel matrix to the
initiator in consideration of the maximum dimension when computing the
transmission weight matrix for beamforming in the initiator, in a case
that the second number is less than the first number; data packet
receiving means for receiving a data packet beamformed in a transmission
weight matrix for beamforming requested from the fed-back channel matrix,
from the initiator; and space division means for multiplying a reception
vector comprising received signals by a reception weight matrix obtained
from the channel matrix, and performing spatial decoding of spatially
multiplexed signals.

16. The wireless communication apparatus according to claim 15, wherein
spatially multiplexed streams are transmitted from the initiator based on
a standard specification, and a capability description field for
describing a maximum spatial dimension of channel information received by
a beamformer in the explicit feedback is defined in a protocol according
to the standard specification; and wherein the notification receiving
means receives a management frame including the capability description
field.

17. The wireless communication apparatus according to claim 16, wherein
the notification receiving means receives a transmission frame, the
transmission frame including at least one of: a beacon signal, which is
notified in a predetermined frame period by the initiator; a measure
pilot; both an association response and a re-association response, which
respond to the request of association from the initiator; and a probe
response which responds to the request of basic service set information
from the initiator, when the initiator is operated as an access point.

18. The wireless communication apparatus according to claim 16, wherein
the notification receiving means receives a transmission frame, the
transmission frame including at least one of: both an association request
and re-association request for requesting network association from the
initiator; and a probe request for requesting basic service set
information to an access point; therein the initiator is operated as a
client terminal in a network and the wireless communication apparatus is
operated as an access point.

19. The wireless communication apparatus according to claim 15, wherein
the notification receiving means receives a maximum spatial dimension
through a packet for requesting the feedback of channel information from
the initiator.

20. (canceled)

21. A wireless communication method for receiving spatially multiplexed
streams from an initiator including a first number of antennas by a
receiver having a second number of antennas, the first number being an
integer equal to or greater than two, and the second number being an
integer equal to or greater than one, the method comprising the steps of:
receiving a maximum dimension when computing a transmission weight matrix
for beamforming of the initiator, the maximum dimension being an integer
equal to or less than the first number; receiving from the initiator a
packet including a training sequence corresponding to the first number of
antennas of the initiator and the second number of the receiver, and
estimating a channel matrix having the second number of rows and the
first number of columns; suppressing the estimated channel matrix to a
third number of rows and the first number of columns, the third number
being equal to or less than the maximum dimension, and feeding back the
suppressed channel matrix to the initiator in consideration of the
maximum dimension when computing the transmission weight matrix for
beamforming in the initiator, in a case that the first number is less
than the second number; receiving a data packet from the initiator
terminal, the data packet being beamformed in the transmission weight
matrix for beamforming obtained from the fed-back channel matrix; and
multiplying a reception vector comprised of received signals by a
reception weight matrix obtained from the channel matrix, and performing
spatial decoding of spatially multiplexed signals.

22-30. (canceled)

31. A wireless communication apparatus including a first number of
antennas for receiving spatially multiplexed streams from an initiator
including a second number antennas, the second number being an integer
equal to or greater than two, and the first number being an integer equal
to or less than one, the apparatus comprising: a notification receiving
unit to receive a maximum dimension when computing a transmission weight
matrix for beamforming of the initiator; a channel matrix estimating unit
to receive a packet including a training sequence corresponding to the
second number of antennas of the initiator and the first number of
antennas of the wireless communication apparatus from the initiator, and
to estimate a channel matrix; a channel information feedback unit to
suppress the channel matrix estimated by the channel matrix estimating
unit to a third number of rows and the second number of columns, the
third number being equal to or less than the maximum dimension, and to
feed back the suppressed channel matrix to the initiator in consideration
of the maximum dimension when computing the transmission weight matrix
for beamforming in the initiator, in a case that the second number is
less than the first number; a data packet receiving unit to receive a
data packet from the initiator, the data packet being beamformed in a
transmission weight matrix for beamforming requested from the fed-back
channel matrix; and a space division unit to multiply a reception vector
comprised of received signals by a reception weight matrix obtained from
the channel matrix, and to perform spatial decoding of a spatially
multiplexed signal.

32-40. (canceled)

Description:

TECHNICAL FIELD

[0001] In one aspect, the invention relates to a wireless communication
system, a wireless communication apparatus, and a wireless communication
method using spatial multiplexing. More particularly, the invention
relates to a wireless communication system, a wireless communication
apparatus, and a wireless communication method, in which a transmitter
and a receiver share channel information to perform closed loop type
spatial multiplexing transmission.

[0002] In another aspect, the invention relates to a wireless
communication system, a wireless communication apparatus, and a wireless
communication method, which perform beamforming by obtaining a channel
matrix on the basis of a training sequence transmitted from a receiver
when a transmitter transmits a packet. More particularly, the invention
relates to a wireless communication system, a wireless communication
apparatus, and a wireless communication method, which perform beamforming
using the training sequence transmitted from the transmitter to the
receiver when the number of antennas of the transmitter, which is a
beamformer, is smaller than that of the receiver, which is a beamformee.

BACKGROUND ART

[0003] Wireless network has attracted much attention recently, because it
is capable of removing wire in an existing wired communication network.
Standard wireless networks include IEEE (Institute of Electrical and
Electronics Engineers) 802.11 or IEEE 802.15.

[0004] For example, IEEE 802.11a/g, a standard of wireless Local Area
Network (LAN), specifies an orthogonal frequency division multiplexing
(OFDM) modulation method, which is a multi-carrier method. Because, in
the OFDM modulation method, transmission data having orthogonal
frequencies is distributed to a plurality of carriers and transmitted,
the band of each carrier becomes narrow, spectrum efficiency is very
high, and resistance to frequency-selective fading interference is
strong.

[0005] In addition, IEEE 802.11a/g standard supports a modulation method
for accomplishing a communication speed up to 54 Mbps. However, a
next-generation wireless LAN standard requires a higher bit rate.

[0006] In order to realize a higher speed for wireless communications,
multi-input multi-output (MIMO) communication has attracted attention.
MIMO communication employs a plurality of antennas in a transmitter and
in a receiver to realize spatially multiplexed streams. The transmitter
performs spatial/temporal encoding and multiplexing of plural pieces of
transmission data, and distributes and transmits the plural pieces of
transmission data to N transmission antennas through channels, where N is
a positive integer. The receiver performs spatial/temporal decoding on
signals received by M reception antennas through the channels to obtain
reception data without crosstalk between the streams (see, for example,
JP-A-2002-44051, hereinafter referred to as Patent Document 1), where M
is a positive integer. Ideally, spatial streams are formed corresponding
to a fewer number of transmission and reception antennas (i.e.,
MIN[N,M]).

[0007] According to MIMO communication, a transmission capacity can be
increased according to the number of antennas, and a communication speed
can be improved without increasing frequency bands. Because spatial
multiplexing is used, spectrum efficiency is high. MIMO communication
uses channel characteristics and is different from a simple
transmission/reception adaptive array. For example, IEEE 802.11n, which
is a standard extended from IEEE 802.11a/g, specifies an OFDM_MIMO method
using OFDM as the primary modulation. Currently, IEEE 802.11n is
standardized in Task Group n (TGn), in which a specification is
established based on a specification established in Enhanced Wireless
Consortium (EWC) formed in October, 2005.

[0008] In MIMO communication, in order to spatially divide a spatially
multiplexed reception signal y into stream signals x, a channel matrix H
is acquired by any method and spatially multiplexed reception signal y
needs to be spatially divided into a plurality of original streams using
channel matrix H by a predetermined algorithm.

[0009] Channel matrix H is obtained by allowing a transmitter/receiver to
transmit/receive existing training sequence, estimating the channels by a
difference between the actually received signal and the existing
sequence, and arranging propagation channels in a matrix form according
to a combination of transmission and reception antennas. When there are N
transmission antennas and M reception antennas, the channel matrix is an
M×N (row times column) matrix. Accordingly, the transmitter
transmits N training sequence and the receiver acquires channel matrix H
using the received training sequence.

[0010] A method for spatially dividing a reception signal is generally
classified into an open loop type method, in which a receiver
independently performs spatial division on the basis of channel matrix H,
and a closed loop type method, in which a transmitter gives weight to
transmission antenna on the basis of channel matrix H to perform adequate
beamforming toward a receiver to form an ideal spatial orthogonal
channel.

[0012] For an ideal closed loop type MIMO transmission method, a singular
value decomposition (SVD)-MIMO method using SVD of channel matrix H is
known (see, for example, http://radio3.ee.uec.ac.jp/MIMO(IEICE_TS).pdf
(Oct. 24, 2003), hereinafter referred to as Non-Patent Document 3). In
the SVD-MIMO transmission, a numerical matrix having channel information
that uses antenna pairs as elements, that is, a channel information
matrix H, is subjected to the singular value decomposition to obtain
UDVH. A transmitter uses V in a transmission antenna weight matrix,
and transmits a beamformed packet to a receiver. A receiver typically
uses (UD)-1 as a reception antenna weight matrix. Here, D is a
diagonal matrix having square roots of singular values λi
corresponding to qualities of the spatial streams in diagonal elements
(the subscript "i" indicates the i-th spatial stream). Singular values
λi are the diagonal elements of diagonal matrix D in ascending
order. Power ratio distribution or modulation method allocation is
performed according to communication quality represented by the level of
singular value with respect to the streams, such that a plurality of
spatial orthogonal multiplexed propagation channels, which are logically
independent, are realized. The receiver can extract a plurality of
original signal sequence without crosstalk, and theoretically accomplish
maximum performance.

[0013] In the closed loop type MIMO communication system, adequate
beamforming is performed when the transmitter transmits a packet, but
information on the channel information needs to be fed back from the
receiver for receiving the packet.

[0014] For example, EWC HT (High Throughput) MAC (Media Access Control)
Specification, Version V1.24, defines two kinds of procedures, "implicit
feedback" and "explicit feedback," as the procedures for feeding back the
information on the channel matrix between the transmitter and the
receiver.

[0015] For "implicit feedback," the transmitter estimates a backward
channel matrix transmitted from the receiver using a training sequence
also transmitted from the receiver. A forward channel matrix transmitted
from the transmitter to the receiver is computed to perform beamforming
under the assumption that bi-directional channel characteristics between
the transmitter and the receiver are reciprocal. Calibration of an RF
circuit in a communication system is performed, such that the channel
characteristics are reciprocal.

[0016] For "explicit feedback," the receiver estimates a forward channel
matrix transmitted from the transmitter using a training sequence also
transmitted from the transmitter, and returns a packet including the
channel matrix as data to the transmitter. The transmitter performs the
beamforming using the received channel matrix. Alternatively, the
receiver computes a transmission weight matrix for allowing the
transmitter to perform the beamforming from the estimated channel matrix,
and returns a packet including the transmission weight matrix as data to
the transmitter. For explicit feedback, because the weight matrix is
computed on the basis of the estimated forward channel matrix, it may not
be assumed that the channels are reciprocal.

[0017] In view of packet transmission, the transmitter is an initiator and
the receiver is a terminator. However, in view of beamforming, the
initiator for transmitting the packet is a beamformer and the terminator
for receiving the beamformed packet is a beamformee. Communication from
the beamformer to the beamformee is referred to as "forward," and
communication from the beamformee to the beamformer is referred to as
"backward." For example, when an access point (AP) transmits a data frame
to a client terminal (STA) as the beamformer, explicit feedback requires
that the access point performs beamforming on the basis of channel
information transmitted from the client terminal.

[0018] For explicit feedback, the beamformer can receive explicit feedback
of the estimation channel matrix from the beamformee. The feedback format
of the estimation channel matrix can generally be classified into two
different cases. In one case, a MIMO channel coefficient is sent; while
in another case, a transmission weight matrix V for beamforming is
computed by the beamformee. The former format is called channel state
information (CSI). The beamformer needs to compute the transmission
weight matrix V for beamforming by constructing the channel matrix H from
received CSI, thereby performing the singular value decomposition. The
latter is further classified into a case where transmission weight matrix
V for beamforming is sent in an uncompressed format, and a case where
transmission weight matrix V for beamforming is sent in a compressed
format. According to the explicit feedback, a processing burden for
estimating the channel matrix in the beamformer and a processing burden
for calculating the transmission weight matrix from the channel matrix
are reduced.

[0019]FIG. 12 shows a frame exchange procedure for transmitting
beamforming from the access point to the client terminal by explicit
feedback.

[0020] This procedure is initiated by the access point which sends a
sounding packet including a CSI feedback request.

[0021] The sounding packet includes the training sequence excited by the
channel matrix. Accordingly, when the sounding packet is received, the
client terminal divides the spatial stream training to estimate channel
matrix H and collects the CSI. CSI data is included in the packet as a
CSI feedback (CFB), and returned to the access point.

[0022] The access point computes the transmission weight matrix for
beamforming from received CFB, and multiplies the transmission signal by
the transmission weight matrix for beamforming to transmit the beamformed
packet to the client terminal. By beamforming, even if the client
terminal is located at a place where wireless communication was difficult
in the past, the client terminal may still perform wireless communication
at a high transmission rate.

[0023] Subsequently, an operation for performing beamforming according to
explicit feedback will be described with reference to FIG. 13. In FIG.
13, a first client terminal STA-A having three antennas is a beamformer,
a second client terminal STA-B having two antennas is the beamformee.
Feedback is performed based on the CSI format. In the following
description or equations, a subscript AB indicates forward transmission
from STA-A to STA-B. A numerical subscript corresponds to antenna number
of the corresponding client terminal.

[0024] The training sequence transmitted from the antennas of STA-A is
(tAB1, tAB2, tAB3) and the signals received by the
antennas of STA-A through a channel HAB are (rAB1, rAB2).
The following equation is obtained.

( r AB 1 r AB 2 ) = H AB (
t AB 1 t AB 2 t AB 3 )
( 1 ) ##EQU00001##

[0025] where, channel matrix HAB is a 2×3 matrix expressed by
equation (2). Here, hij is a channel characteristics value of the
j-th antenna of STA-A to the i-th antenna of STA-B.

H AB = ( h 11 h 12 h 13 h 21 h 22 h 23
) ( 2 ) ##EQU00002##

[0026] When channel matrix HAB is subjected to singular value
decomposition, equation (3) is obtained. Here, UAB is a matrix
having an inherent normalized vector of HABHABH, VAB
is an inherent normalized vector of HABHHAB, and DAB
is a diagonal matrix having a square root of an inherent vector of
HABHABH or HABHHAB as the diagonal
elements. In addition, UAB and VAB are unitary matrices, namely
complex conjugates of transposed matrices become the inverse of the
matrices.

HAB=UABDABVABH (3)

[0027] The transmission weight matrix necessary for forming the frame
transmitted from STA-A to STA-B is matrix VAB obtained by performing
the singular value decomposition with respect to forward channel matrix
HAB. When the beamformee receives a sounding packet, the beamformee
divides the sounding packet into spatial stream trainings to construct
estimation channel matrix HAB. The CSI composed of MIMO channel
coefficients h11, h12, etc., which are elements of the channel
matrix is collected and fed back to STA-A.

[0028] If a transmission vector composed of transmission signals of the
antennas of STA-A is x, and a reception signal of STA-B is y, the
reception signal becomes y=HABx in a case where the beamforming is
not performed (un-steered), but reception signal y becomes equation (4)
in a case where the beamforming are performed by transmission weight
matrix VAB (steered).

[0029] Accordingly, STA-B can perform spatial division to the original
stream by multiplying a reception vector including the reception signals
of the antennas by DAB-1UABH as a reception weight.

[0030] When beamforming according to the explicit feedback is performed
with the CSI format, there is a reduced burden on a process of estimating
the channel matrix in the beamformer. However, the terminal, which is the
beamformer, computes the transmission weight matrix for beamforming by
performing the singular value decomposition, or other calculation
methods, with respect to the channel matrix fed back from the beamformee.
This is a heavily loaded calculation and the load increases depending on
the dimension of the channel matrix.

[0031] In an example shown in FIG. 13, STA-A includes three antennas
(N=3), and STA-B includes two antennas (M=2). Because there are more
antennas in STA-A than in STA-B, no problem is caused in the processing
capability for beamforming. This is because STA-A is designed to include
the processing capability corresponding to N of its own streams; and an
N×M channel matrix is constructed on the basis of the CSI fed back
from the beamformee to perform computation of the matrix for beamforming
on the basis of the channel matrix.

[0032] However, for N<M, that is, the number of antennas of the
beamformee is larger than that of the beamformer, problems may be caused
because the beamformer does not include the processing capability which
exceeds the number of its own spatial streams. When STA-A can process
only N streams, which is equal to the number of antennas, the matrix for
beamforming may not be obtained from the N×M estimation channel
matrix.

[0033] In order to solve such a problem without deteriorating the
beamforming characteristics, it may be considered that a channel
estimation maximum dimension Mmax corresponding to a rated maximum
number of antennas is given to STA-A as the beamformee (for example, if
it is based on the IEEE specification, Mmax=4), and a processing
capability for computing the transmission weight matrix for beamforming
is given to the obtained Mmax×N estimation channel matrix.

[0034] For example, when STA-A includes two antennas (i.e. N=2) and the
rated maximum number of antennas is Mmax=4, STA-A can compute only a
2×2 matrix for communication with the terminal having the same
number of antennas, but must compute a 4×2 matrix. In this case,
calculation or processing circuit needs to be doubled, which renders it
difficult to reduce the size and the cost of the communication apparatus.

DISCLOSURE OF INVENTION

[0035] There are provided a wireless communication system, a wireless
communication apparatus, and a wireless communication method, which are
capable of performing communication at a high transmission rate by a
beamformed packet by allowing a terminal, which is operated as a
beamformer, to obtain a transmission weight matrix on the basis of an
estimation channel matrix fed back from a terminal, which is operated as
a beamformee.

[0036] There are also provided a wireless communication system, a wireless
communication apparatus, and a wireless communication method, which are
capable of performing beamforming by explicit feedback without
deteriorating beamforming characteristics, nor increasing a processing
capability of channel estimation or a computing capability of a matrix
for beamforming in the beamformer, even when the number of antennas of a
terminal, which is a beamformer, is smaller than that of a beamformee.

[0037] According to a first embodiment, there is provided a wireless
communication system for transmitting spatially multiplexed streams from
a first terminal including N antennas to a second terminal including M
antennas (N is an integer of 2 or more and M is an integer of 1 or more),
the system including: notifying means for notifying the second terminal
of a maximum dimension Mmax at the time of computing a transmission
weight matrix for beamforming of the first terminal (Mmax is an
integer of N or less); training means for transmitting a packet including
training sequence corresponding to the number N of antennas of the first
terminal and the number M of antennas of the second terminal from the
first terminal to the second terminal; channel matrix estimation means
for dividing the training sequence received by the antennas of the second
terminal into M streams, and estimating a channel matrix; channel
information feedback means for suppressing a dimension number of the
channel matrix estimated by the second terminal to Mmax or less rows
and N columns, and feeding back the suppressed channel matrix to the
first terminal in consideration of the maximum dimension Mmax at the
time of computing the transmission weight matrix for beamforming in the
first terminal, in a case of N<M; transmission weight matrix
computation means for obtaining the transmission weight matrix for
beamforming at the time of transmitting data from the first terminal to
the second terminal using the channel matrix having Mmax or less
rows and N columns fed back from the second terminal to the first
terminal; and beamforming means for performing beamforming in
transmission signals of the antennas of the first terminal using the
transmission weight matrix for beamforming when a data packet is
transmitted from the first terminal to the second terminal.

[0038] The term "system" described herein indicates a logical set of
apparatuses (or function modules for realizing specific functions). It is
appreciated that the apparatuses or the function modules are not
necessarily included in a single housing. The same is true in the
following descriptions.

[0039] In order to realize high speed wireless communication, there is
provided a MIMO communication method, which employs a plurality of
antenna elements in a transmitter and a receiver, thereby communicating
via spatially multiplexed streams. In particular, in a closed loop type
MIMO communication system, a terminal that transmits a data packet
performs beamforming on the basis of feedback of information on an
estimation channel matrix from a receiving terminal, such that a
plurality of spatial orthogonal multiplexed propagation channels, which
are logically independent, are realized, and the receiving terminal can
extract a plurality of original signal sequence without crosstalk,
thereby theoretically accomplishing a maximum performance.

[0040] In order to perform feedback of the channel matrix from the
receiving terminal to the transmitting terminal, for example, two kinds
of procedures, that is, "implicit feedback" and "explicit feedback," are
defined in the EWC HT MAC specification. Among them, in the explicit
feedback, the first terminal, which is operated as a beamformer, performs
beamforming of a transmission packet using the transmission weight matrix
for beamforming based on the channel information fed back from the second
terminal, which is operated as a beamformee.

[0041] When the beamforming according to the explicit feedback is
performed with the CSI format, there is a reduced burden on estimating
the channel matrix in the beamformer.

[0042] However, for N<M, that is, the number of antennas of the
beamformee is larger than that of the beamformer, problems may arise,
because the beamformer does not include the processing capability, which
exceeds the number of its own spatial streams. When terminal STA-A can
process only N streams, which is equal to the number of antennas, the
matrix for beamforming may not be obtained from the M×N estimation
channel matrix.

[0043] In the wireless communication system according to a first
embodiment, when beamforming is performed according to the explicit
feedback, the maximum dimension Mmax for computing the transmission
weight matrix for beamforming of the first terminal is pre-notified to
the second terminal, and the second terminal transmits the packet
including the training sequence for exciting the Mmax×N
forward channel matrix in correspondence with the maximum dimension
Mmax of the matrix operation of the first terminal and the number N
of antennas of the first terminal. In other words, the second terminal
suppresses the dimension of the estimation matrix to be less than or
equal to the maximum dimension Mmax of the matrix operation of the
first terminal, and returns the CSI information. Accordingly, the first
terminal can obtain the transmission weight matrix for beamforming in a
range of the processing capability corresponding to the number of its own
antennas.

[0044] Accordingly, in one embodiment, when a closed loop type MIMO
communication is performed by the explicit feedback, the channel
estimation, in which the dimension number is suppressed according to the
number of antennas of the beamformer, is fed back from the beamformee.
Thus, the first terminal, which is operated as the beamformer, can
perform the computation of the transmission weight matrix for
beamforming, in which the dimension number is suppressed, thereby
reducing the circuit size of the first terminal.

[0045] In more detail, compared with a case where an M×N channel
matrix is fed back, the circuit size of a buffer part for receiving the
CSI information can be reduced to an order of (N/M)2. In addition,
because the transmission weight matrix for beamforming is computed from
an N×N channel matrix, the circuit size of the beamforming
transmission weight matrix computation unit can be reduced to an order of
(N/M)2, compared with the case where the transmission weight matrix
is computed from the M×N channel matrix. Due to the reduction of
circuit size, it is possible to reduce power consumption of the
apparatus.

[0046] Because the CSI information fed back in the channel is reduced from
M×N to N×N, overhead is reduced, thereby improving the
overall throughput of the system.

[0047] Due to the reduction of circuit size and the reduction of channel
overhead, it is possible to reduce delay related to a communication
process and to reduce time for applying the beamforming, thereby
performing the beamforming based on fresh channel information. It is
possible to minimize the deterioration of characteristics by the
beamforming according to the fresh channel information.

[0048] In one embodiment, means for notifying the maximum dimension
Mmax at the time of computing the transmission weight matrix for
beamforming of the first terminal to the second terminal is not specially
limited.

[0049] For example, in the EWC specification, it is defined that any HT
function supported by a HT terminal is transmitted as the HT capability
element, and is declared. In the HT capability element, a transmit
beamforming (TxBF) capability field for describing the existence of
support of any HT function for beamforming is provided. Accordingly, when
the terminal operated as the beamformee performs the explicit feedback, a
capability description field for describing the spatial dimension number
of the sounding packet, which can be received from the beamformer, is
included. A field for describing the spatial dimension number allowed to
the CSI information when the beamformer performs the explicit feedback
may be further defined in the capability description field.

[0050] The HT capability element may be included in a predetermined
management frame. For example, when the wireless communication apparatus
is operated as an access point, the HT capability field may be included
in a type of transmission frame. The transmission frame may be a beacon
which is notified in a frame period, a measure pilot, an association
response and a re-association response which respond to the request of
association from the client terminal, or a probe response which responds
to the request of BBS information from the client terminal. When the
wireless communication apparatus is operated as a client terminal (or a
communication station other than the access point), the HT capability
field may be included in a type of transmission frame of an association
request and re-association request for requesting network association to
the access point, and a probe request for requesting BSS information to
the access point. Accordingly, even when the wireless communication
apparatus is operated as either an access point or a client terminal, the
wireless communication apparatus can notify the beamformee of the maximum
dimension number allowed to the CSI information as the beamformer by
transmitting the HT capability element.

[0051] Alternatively, it is considered that the beamformer specifies the
maximum spatial dimension of the CSI information in the packet for
requesting the CSI information to the beamformee. For example, the
CSI/Steering field for requesting the CSI information is provided in the
HT control field of the MAC frame defined in the EWC specification, and a
packet transmission source can request the CSI information in the packet
unit. Accordingly, a field for describing the spatial dimension number
allowed to the CSI information may be further defined in the HT control
field.

[0052] The beamformer may include the signal for requesting the CSI
information in the sounding packet including the training sequence for
exciting the channel.

[0053] In the EWC specification, a zero length frame (ZLF) (also called a
null data packet (NDP), hereinafter referred to as "ZLF") dedicated to
the sounding packet is defined. The ZLF includes only a PHY header part
including the training sequence for exciting the channel and does not
include an MAC frame. Because the ZLF does not have the MAC header, the
CSI information cannot be requested by the HT control field. In such a
case, the training means does not include the signal for requesting the
CSI information in the sounding packet and requests the CSI information
in the HT control field of a general packet previously transmitted
thereto. The maximum spatial dimension number of the CSI information is
specified in the general packet.

[0054] According to a second embodiment, there is provided a wireless
communication system which performs data transmission using spatially
multiplexed streams from a first terminal including N antennas to a
second terminal including M antennas (N is an integer of 2 or more and M
is an integer of 1 or more), the system including: training means for
transmitting a packet including training sequence corresponding to the
number N of antennas of the first terminal and the number M of antennas
of the second terminal from the first terminal to the second terminal;
channel matrix estimation means for dividing the training sequence
received by the antennas of the second terminal into M streams and
estimating an M×N channel matrix; channel information feedback
means for feeding back the M×N channel matrix estimated in the
second terminal to the first terminal; transmission weight matrix
computation means for obtaining a transmission weight matrix for
beamforming at the time of transmitting data from the first terminal to
the second terminal in an N×N range of the M×N channel matrix
fed back from the second terminal to the first terminal, in consideration
of the number N of antennas of the first terminal; and beamforming means
for performing beamforming in transmission signals of the antennas of the
first terminal using the transmission weight matrix for beamforming when
a data packet is transmitted from the first terminal to the second
terminal.

[0055] In the wireless communication system according to the second
embodiment, the beamforming performed according to the explicit feedback
is different from that of the first embodiment. The primary difference is
in that a procedure of notifying the second terminal of the maximum
dimension Mmax for computing the transmission weight matrix for
beamforming of the first terminal is omitted in the second embodiment. In
this case, the second terminal estimates an M×N channel matrix and
feeds back the channel matrix without any changes, and the first terminal
obtains the transmission weight matrix for beamforming in an N×N
range of the M×N channel matrix in consideration of the number N of
its own antennas. That is, the first terminal can perform the computation
of the transmission weight matrix for beamforming in the range of the
processing corresponding to the number of its own antennas to reduce the
circuit size of the first terminal.

[0056] In the wireless communication system according to the second
embodiment, the circuit size of the buffer part for receiving the CSI
information cannot be reduced, or the overhead due to the feedback of the
CSI information cannot be reduced, because M×N channels are fed
back, but the transmission weight matrix for beamforming is computed from
the N×N channel matrix. Accordingly, the circuit size of the
beamforming transmission weight matrix computation unit can be reduced to
an order of (N/M)2, compared with the case where the transmission
weight matrix is computed from the M×N channel matrix. Due to the
reduction in circuit size, it is possible to reduce power consumption of
the apparatus. Due to the reduction in circuit size, it is possible to
reduce delay related to a communication process and to reduce time for
applying the beamforming and thus to perform the beamforming based on
fresh channel information. It is possible to significantly suppress the
deterioration of characteristics by the beamforming according to the
fresh channel information.

[0057] According to the embodiment, there are provided a wireless
communication system, a wireless communication apparatus, and a wireless
communication method, which are capable of performing communication at a
high transmission rate by a beamformed packet by allowing a terminal,
which is operated as a beamformer to suitably set a transmission weight
matrix on the basis of an estimation channel matrix fed back from a
terminal, which is operated as a beamformee.

[0058] According to the embodiment, there are provided a wireless
communication system, a wireless communication apparatus, and a wireless
communication method, which are capable of suitably performing
beamforming without increasing a processing capability of channel
estimation, or a computing capability of a matrix for beamforming in the
beamformer, even when the number of antennas of a terminal, which is a
beamformer, is smaller than that of a beamformee.

[0059] In the wireless communication system according to the embodiment,
when beamforming is performed on the basis of a backward channel
estimation result by the explicit feedback, and the number of antennas of
a terminal of a transmitter side is smaller than that of a terminal of a
receiver side, a transmission weight matrix for beamforming in which the
dimension is suppressed can be computed by previously notifying of a
spatial dimension number in the terminal of the transmitter side, thereby
reducing the circuit size of the terminal of the transmitter side.

[0060] According to the embodiment, in the explicit feedback, it is
possible to reduce the circuit size of the apparatus operated as the
beamformer, thereby reducing power consumption of the apparatus, by
suppressing the dimension number of the channel matrix fed back from the
beamformee, or suppressing the dimension number of the computation of the
transmission weight matrix for beamforming in the beamformer.

[0061] Because the CSI information fed back from the beamformee to the
beamformer is reduced from M×N to N×N, overhead is reduced,
thus improving the overall throughput of the system.

[0062] Due to the reduction in circuit size and the reduction of channel
overhead, it is possible to reduce delay related to a communication
process and to reduce time for applying the beamforming, and thus to
perform the beamforming based on fresh channel information. It is
possible to minimize the deterioration of characteristics by the
beamforming according to the fresh channel information.

[0063] Other features and/or advantages of the invention will become
apparent and more readily appreciated from the following description of
the embodiments, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0064] FIG. 1 is a schematic diagram illustrating an operation procedure
of explicit feedback, according to an embodiment consistent with the
invention.

[0065]FIG. 2 illustrates a transmitter of a wireless communication
apparatus, which can be operated as an STA-A (or STA-B) shown in FIG. 1.

[0066]FIG. 3 illustrates a receiver of the wireless communication
apparatus, which can be operated as the STA-A (or STA-B) shown in FIG. 1.

[0071]FIG. 8A shows an example of a transmission operation of a ZLF
packet.

[0072]FIG. 8B shows an example of the transmission operation of the ZLF
packet.

[0073]FIG. 9 illustrates a method of dividing a spatial stream training
from a sounding packet transmitted from STA-A, thereby estimating a
channel matrix.

[0074]FIG. 10 is a flowchart illustrating a process when operating the
wireless communication apparatuses shown in FIGS. 2 and 3 as a beamformer
on the basis of the explicit feedback.

[0075]FIG. 11 is a flowchart illustrating a process when operating the
wireless communication apparatuses shown in FIGS. 2 and 3 as a beamformee
on the basis of the explicit feedback.

[0076]FIG. 12 illustrates a frame exchange procedure for transmitting
beamforming from an access point to a client terminal by explicit
feedback.

[0077] FIG. 13 is a view illustrating a calculation process for performing
the beamforming according to the explicit feedback.

[0078]FIG. 14 is a view showing an aspect of using two bits of B25 to B26
of a Tx beamforming capability field as a "maximum CSI dimension at
beamformer" field.

BEST MODE FOR CARRYING OUT THE INVENTION

[0079] Hereinafter, embodiments will be described in detail with reference
to the accompanying drawings.

[0080] A wireless communication system according to an embodiment may
perform closed loop type MIMO communication. Particularly, a transmitter
terminal may perform beamforming in order to perform feedback for a
channel matrix, for example, the "explicit feedback" defined in the EWC
HT MAC specification. For explicit feedback, a beamformer performs
beamforming on a transmission packet using a transmission weight matrix
for beamforming obtained on the basis of an estimation channel matrix fed
back from a beamformee, so as to establish communication.

[0081] However, a terminal has a processing capability for performing
channel estimation, or a processing capability for computing a matrix for
beamforming. The processing capability depends largely on the number of
antennas that the terminal includes. Accordingly, if the beamformer
includes a large number of antennas, the beamformer may not obtain a
matrix for beamforming, because the spatial dimension number of the
matrix is large even when an estimation channel matrix is fed back from
the beamformee.

[0082] In the wireless communication system, according to the embodiment,
when beamforming is performed according to explicit feedback, the
beamformee is pre-notified of a maximum dimension Mmax for computing
the transmission weight matrix for beamforming of the beamformer. The
beamformee transmits the packet including a forward channel matrix
information having Mmax or fewer rows and N columns in
correspondence with maximum dimension Mmax of the matrix operation
of the beamformer. In other words, the beamformee suppresses the
dimension number of the estimation channel matrix to be less than or
equal to maximum dimension Mmax and returns the CSI information.
Accordingly, the beamformer can obtain the transmission weight matrix for
beamforming in a range of the processing capability corresponding to the
number of its own antenna.

[0083] FIG. 1 is a schematic diagram illustrating an operation procedure
of explicit feedback according to an embodiment. Here, a terminal STA-A,
which is operated as a beamformer, includes a first number of antennas.
In this particular embodiment, the first number is two, which is equal to
a maximum dimension number when computing the transmission weight matrix
for beamforming. A terminal STA-B, which is operated as a beamformee,
includes a second number of antennas. In this particular embodiment, the
second number is three. This procedure is carried out under the basis of
the EWC MAC specification.

[0084] First, terminal STA-A transmits a sounding packet including a
training sequence to terminal STA-B, and performs a CSI request in
explicit feedback. Terminal STA-A includes information of the maximum
dimension number when computing the transmission weight matrix for
beamforming in the CSI request. Terminal STA-B is pre-notified of the
maximum dimension number by a separate procedure.

[0085] The sounding packet transmitted from terminal STA-A excites a
3×2 forward channel matrix. Terminal STA-B is designed to include
processing capability corresponding to the number of its own streams.
When the sounding packet is received, the 3×2 forward channel
matrix can be generated without a problem.

[0086] Terminal STA-B suppresses the dimension number of CSI information
for feeding back the generated estimation channel matrix to
Mmax×N or less in consideration of the processing capability
of terminal STA-A. The feedback of CSI information is suitable when a
fewer number of spatial streams is used either in the transmission
capability of terminal STA-A or in a reception capability of terminal
STA-B. For example, only one or two spatial streams is used.

[0087] When receiving the CSI information, terminal STA-A can compute the
transmission weight matrix for beamforming in the processing capability,
thereby reducing the circuit size of the terminal.

[0088] Thereafter, the request of sounding packet, the channel estimation
due to the reception of sounding packet, and the computation of the
transmission weight matrix for beamforming are repeatedly performed
whenever terminal STA-A performs the beamforming.

[0089] Because the dimension number of the channel estimation is
suppressed according to the number of antennas of terminal STA-A, the
channel estimation is fed back from terminal STA-B. Terminal STA-A, which
is operated as the beamformer, can obtain the transmission weight matrix
for beamforming in which the dimension number is suppressed, thereby
reducing the circuit size of terminal STA-A.

[0090] More specifically, in one example, if Mmax=N, the circuit size
of a buffer part for receiving the CSI information can be reduced to an
order of (N/M)2, compared with a case where an M×N channel
matrix is fed back (in this case, N=2 and M=3). In addition, because the
transmission weight matrix for beamforming is computed from an N×N
channel matrix, the circuit size of the beamforming transmission weight
matrix computation unit can be reduced to an order of (N/M)2,
compared with the case where the transmission weight matrix is computed
from the M×N channel matrix. Due to the reduction in circuit size,
it is possible to reduce power consumption of the apparatus.

[0091] Because the CSI information fed back in the channel is reduced from
M×N to N×N, overhead is reduced and thus the overall
throughput of the system can be improved.

[0092] Due to the reduction in circuit size and the reduction in channel
overhead, it is possible to reduce delay related to a communication
process and to reduce time for applying the beamforming, thereby
performing the beamforming based on fresh channel information. It is
possible to minimize the deterioration of beamforming characteristics
according to the fresh channel information.

[0093] Compared with a case where the beamforming is performed using the
M×N estimation channel matrix, the characteristics may be
deteriorated, but the fresh channel information can be applied for a
short time due to the overhead reduction. Accordingly, the deterioration
may be minimized.

[0094] In order to realize the above-described beamforming procedure,
channel estimation maximum dimension Mmax of the beamformer needs to
be notified to the beamformee.

[0095] For example, in the EWC specification, it is defined that any HT
function supported by a HT terminal is transmitted as the HT capability
element and is declared. In the HT capability element, a transmit
beamforming (TxBF) capability field for describing the existence of the
support of any HT function for beamforming is provided.

[0096]FIG. 4 shows a format of the HT capability element. In the TxBF
capability field, HT function of the beamforming is specified. FIG. 5
shows the configuration of the Tx beamforming capability field.

[0097] The Tx beamforming capability field has 32 bits, but, among them,
19th to 20th bits are allocated to the CSI number of beamformer
antennas, 21st to 22nd bits are allocated to the uncompressed
steering matrix of beamformer antennas, and 23rd to 24th bits
are allocated to the compressed steering matrix of beamformer antennas.
In these fields, the spatial dimension number of the sounding packet
which can be received from the beamformer when the beamformee performs
the explicit feedback with each format is described. Afield for
describing the spatial dimension number allowed to the CSI information
when the beamformer performs the explicit feedback may be further defined
in the field. As an additional defining method, for example, information
on the maximum spatial dimension when receiving the sounding packet is
described using a partial bit field of B25 to B31 which is a "reserved"
area in the current Tx beamforming capability field. In particular, two
bits of B25 to B26 are used as "maximum CSI dimension at beamformer"
field (see FIG. 14). A matrix having one row and N columns is defined as
a maximum, if the value thereof is zero; a matrix having two rows and N
columns is defined as a maximum, if the value thereof is one; a matrix
having three rows and N columns is defined as a maximum, if the value
thereof is two; and a matrix having four rows and N columns is defined as
a maximum, if the value thereof is three, thereby representing the
spatial dimension number allowed to the CSI information when receiving
the sounding packet.

[0098] The HT capability element may be included in a predetermined
management frame. For example, when terminal STA-A is operated as the
access point, the HT capability field may be included in a transmission
frame. The transmission frame maybe one of: the beacon, which is notified
in each frame period, a measure pilot; both an association response and a
re-association response, which respond to the request of association from
the client terminal; and a probe response, which responds to the request
of Basic Service Set (BSS) information from the client terminal, such
that the dimension number of CSI information is notified to terminal
STA-B, which participates in the network operated by terminal STA-A. When
terminal STA-A is operated as the client terminal (or a communication
station other than the access point), the HT capability field may be
included in a transmission frame. The transmission frame may be one of:
both an association request and re-association request for requesting
network association to terminal STA-B, which is operated as the access
point; and a probe request for requesting BSS information to the access
point. Accordingly, even when terminal STA-A is operated as either the
access point or the client terminal, terminal STA-A can notify terminal
STA-B of the maximum dimension number allowed to the CSI information by
transmitting the HT capability element.

[0099] The CSI dimension information described in the HT capability
element transmitted from terminal STA-A is efficient in a terminal other
than terminal STA-B. For example, when terminal STA-A performs implicit
beamforming using the CSI feedback with respect to a terminal STA-C (not
shown), the CSI dimension information is not sent again.

[0100] Alternatively, it is considered that the beamformer specifies the
maximum spatial dimension of CSI information in the packet for requesting
CSI information to the beamformee. FIG. 6 schematically shows a HT
control field of the MAC frame defined in the EWC specification. The HTC
field has 32 bits. Among them, in the CSI/steering field at the 22nd
to the 23rd bits, a packet transmission source may request the CSI
information in the packet unit. A field for describing the spatial
dimension number allowed to the CSI information may be further defined in
the HTC field.

[0101] Subsequently, a modified example of the operation procedure of the
explicit feedback will be described with reference to FIG. 1.

[0102] First, terminal STA-A transmits a sounding packet including the
training sequence to terminal STA-B, and performs a CSI request in the
explicit feedback. However, terminal STA-A is not notified of the
information on the maximum dimension number at the time of computing the
transmission weight matrix for beamforming.

[0103] The sounding packet transmitted from terminal STA-A excites a
3×2 forward channel matrix. Terminal STA-B is designed to include
processing capability corresponding to the number of its own streams.
When the sounding packet is received, the 3×2 forward channel
matrix can be generated without a problem.

[0104] Terminal STA-B feeds back the 3×2 channel matrix to terminal
STA-A as the CSI information without change. The feedback of CSI
information is suitable when a fewer number of spatial streams is used
either in the transmission capability of the STA-A or in a reception
capability of the STA-B. For example, only one or two spatial streams is
used.

[0105] Terminal STA-A requests the transmission weight matrix for
beamforming in a 2×2 range of the 3×2 channel matrix in
consideration of the number of its own antennas.

[0106] Thereafter, the request of the sounding packet, the channel
estimation due to the reception of the sounding packet, and the
computation of the transmission weight matrix for beamforming are
repeatedly performed whenever terminal STA-A performs the beamforming.

[0107] Because terminal STA-A obtains the transmission weight matrix for
beamforming by the dimension number in the range of its own number of
antennas, it is possible to reduce the circuit size.

[0108] In this case, terminal STA-A does not reduce the circuit size of
the buffer part for receiving the CSI information or the feedback
overhead of the CSI information, but computes the transmission weight
matrix for beamforming from the N×N channel matrix if Mmax=N
as a typical example. Accordingly, the circuit size of the beamforming
transmission weight matrix computation unit can be reduced to an order of
(N/M)2 (in this case, N=2 and M=3), compared with the case where the
transmission weight matrix is computed from the M×N channel matrix.

[0109] Due to the reduction in circuit size, it is possible to reduce
power consumption of the apparatus. Due to the reduction in circuit size,
it is possible to reduce delay related to a communication process and to
reduce time for applying the beamforming, thereby performing the
beamforming based on fresh channel information. It is possible to
significantly suppress the deterioration of characteristics by the
beamforming according to the fresh channel information.

[0110] In the beamforming procedure shown in FIG. 1, terminal STA-A, which
is operated as the beamformer, includes the signal for requesting the CSI
information in the sounding packet, the sounding packet including the
training sequence for exciting the channel. In more detail, in the
CSI/steering field provided in the HT control field of the MAC frame, a
feedback method received from the beamformee in the explicit feedback can
be specified (see FIG. 7).

[0111] In the EWC specification, a zero length frame (ZLF) dedicated to
the sounding packet is defined. The ZLF includes only a PHY header part
including the training sequence for exciting the channel, and does not
include an MAC frame. Because the ZLF does not have the MAC header, the
CSI information cannot be requested by the HT control field. In such a
case, the training means does not include the signal for requesting the
CSI information in the sounding packet. Instead, the training means
requests the CSI information in the HT control field of a general packet
transmitted prior to the sounding packet.

[0112]FIG. 8A shows an example of a transmission operation of the ZLF
packet. As shown, the ZLF packet is transmitted when a short interframe
space (SIFS) or a reduced inter frame space (RIFS) elapses after a
general data packet is transmitted. In the HT control field in the MAC
header included in the general data packet, the CSI request for the
subsequent ZLF packet is performed by specifying the CSI/Steering field.

[0113] In an example shown in FIG. 8B, terminal STA-A requests the
feedback of the CSI information in the data frame for requesting an
immediate response, but declares that the ZLF is continuously transmitted
therein. When terminal STA-B returns an ACK according to the immediate
response, terminal STA-A transmits the ZLF when the SIFS elapses after
the ACK is received.

[0114] FIGS. 2 and 3 show the configurations of the transmitter and the
receiver of a wireless communication apparatus, which can be operated as
terminal STA-A (or terminal STA-B) in the wireless communication system
shown in FIG. 1, respectively. The number of antennas of terminal STA-A
is N, while the number of antennas of terminal STA-B is M. Here, N or M
is at most four, for example, on the basis of the IEEE specification, but
only two antennas are shown in the figures in order to avoid conflict of
the figures.

[0115] Transmission data supplied to a data generator 100 is scrambled by
a scrambler 102. Subsequently, error correction encoding is performed by
an encoder 104. For example, in the EWC HT PHY specification, the
scrambling and encoding methods are defined according to the definition
of IEEE 802.11a. The encoded signal is input to a data division unit 106
to be divided into transmission streams.

[0116] In a case where the apparatus is operated as the beamformer, data
generator 100 generates an MAC frame for describing the request of CSI
information when performing the explicit feedback. In a case where the
apparatus is operated as the beamformee, a channel matrix estimation unit
216a of the receiver constructs a data frame including the CSI
information on the basis of the estimated channel matrix, in response to
the reception of the CSI information request.

[0117] In each transmission stream, the transmission signal is punctured
by a puncture 108 according to a data rate applied to each stream,
interleaved by an interleaver 110, mapped to an IQ signal space by a
mapper 112, thereby becoming a conjugate baseband signal. In the EWC HT
PHY specification, an interleaving scheme expands the definition of IEEE
802.11a, such that the same interleaving is not performed among a
plurality of streams. As the mapping scheme, BPSK, QPSK, 16QAM, or 64QAM
is applied according to IEEE 802.11a.

[0118] A selector 111 inserts the training sequence into the transmission
signal of each interleaved spatial stream at an adequate timing and
supplies it to mapper 112. The training sequence includes the HT-STF
(short training field) for improving the AGC in the MIMO system and the
HT-LTF (long training field) for performing the channel estimation for
each input signal, which is spatially modulated in the receiver.

[0119] When beamforming is performed with respect to the transmission
signal, in a spatial multiplexer 114, a beamforming transmission weight
matrix computation unit 114a calculates transmission weight matrix V for
beamforming from channel matrix H using a computation method such as the
singular value decomposition. A transmission weight matrix multiplication
unit 114b multiplies the transmission vector having the transmission
streams as the element by the transmission weight matrix V, thereby
performing the beamforming. When transmitting the sounding packet, the
beamforming is not performed with respect to the transmission signal.

[0120] When the explicit feedback using the CSI format is performed,
beamforming transmission weight matrix computation unit 114a computes the
transmission weight matrix using the forward channel matrix constructed
based on the CSI information fed back from the beamformee. When the CSI
dimension information is notified to the beamformee as the maximum
dimension number computed by beamforming transmission weight matrix
computation unit 114a, the CSI information returned from the beamformee
is the channel information, in which the dimension number is suppressed
to Mmax×N. When the CSI dimension information is not notified
to the beamformee, the CSI information returned from the beamformee
becomes the M×N channel matrix estimated by the beamformee. In the
latter case, the beamforming transmission weight matrix computation unit
114a extracts only Mmax rows from the M×N matrix, constructs
an Mmax×N forward channel matrix, and performs the singular
value decomposition with respect to the Mmax×N forward channel
matrix to obtain the transmission weight matrix V. In either case, the
circuit size of the beamforming transmission weight matrix computation
unit can be reduced to an order of (Mmax/M)2, compared with the
case where the transmission weight matrix is computed from the M×N
channel matrix.

[0121] An inverse fast Fourier transform unit (IFFT) 116 converts the
subcarriers arranged in a frequency domain into a time domain signal. A
guard insertion unit 118 adds a guard interval. A digital filter 120
performs band limitation, a Digital-Analog converter (DAC) 122 converts
the band-limited signal into an analog signal, and an RF unit 124
up-converts the analog signal to an adequate frequency band and transmits
the converted signal to the channel through each transmission antenna.

[0122] Meanwhile, the data which reaches the receiver through the channel
is analog-processed in an RF unit 228, converted into a digital signal by
an Analog-Digital converter (ADC) 226, and input to a digital filter 224,
in each reception antenna branch.

[0124] A space division unit 216 performs a space division process of the
spatially multiplexed reception signal. In particular, a channel matrix
estimation unit 216a divides the spatial stream training included in the
PHY header of the sounding packet and constructs an estimation channel
matrix H from the training sequence.

[0125] An antenna reception weight matrix computation unit 216b computes
an antenna reception weight matrix W on the basis of channel matrix H
obtained by channel matrix estimation unit 216a. In a case where the
beamforming is performed with respect to the reception packet and the
estimation channel matrix is subjected to the singular value
decomposition, the estimation channel matrix becomes equal to an UD (see
Equation 3) and antenna reception weight W is calculated therefrom. A
method for calculating antenna reception weight W is not limited to the
singular value decomposition. Other calculation methods, such as zero
forcing and MMSE, may be used. An antenna reception weight matrix
multiplication unit 216c multiplies the reception vector having the
reception streams as the element by antenna reception weight matrix W to
perform spatial decoding of the spatial multiplexed signal, thereby
obtaining independent signal sequence for each stream.

[0126] For explicit feedback, when the apparatus is operated as the
beamformee, the CSI information is constructed from estimation channel
matrix H obtained by channel matrix estimation unit 216a, and fed back
from the transmitter to the beamformer as the transmission data. When the
CSI dimension information is notified as the maximum dimension number, in
which the beamformer can compute the transmission weight matrix for
beamforming, the N×N channel matrix, in which the dimension number
is suppressed according to the CSI dimension information, is fed back as
the CSI information. When the CSI dimension information is not notified,
the CSI information is constructed from M×N estimation channel
matrix H obtained by channel matrix estimation unit 216a without any
changes.

[0127] For example, as shown in FIG. 9, if terminal STA-A includes two
antennas (i.e. N=2), terminal STA-B includes three antennas (i.e. M=3),
and the wireless communication apparatus is operated as terminal STA-B,
that is, the beamformee, forward channel matrix H obtained by the channel
matrix estimation unit 216a becomes a 3×2 matrix as expressed by
Equation 5.

H AB = ( h 11 h 12 h 21 h 22 h 31 h 32
) ( 5 ) ##EQU00004##

[0128] When N=2 is notified from terminal STA-A, which is operated as the
beamformer, as the CSI dimension information, the CSI information is
constructed using a 2×2 channel matrix formed by extracting two
rows from the 3×2 channel matrix.

[0129] A channel equalization circuit 214 performs remaining frequency
offset correction and channel tracking with respect to the signal
sequence of each stream. A demapper 212 demaps the reception signal on
the IQ signal space, a deinterleaver 210 performs deinterleaving, and a
depuncture 208 performs depuncturing at a predetermined data rate.

[0130] A data synthesis unit 206 synthesizes a plurality of reception
streams to one stream. This data synthesis process performs an operation,
which is opposed to the data division performed in the transmitter. A
decoder 204 performs error correction decoding, a descrambler 202
performs descrambling, and a data acquiring unit 200 acquires the
reception data.

[0131] When the apparatus is operated as the beamformer, the CSI
information acquired by data acquiring unit 200 is sent to transmission
weight matrix computation unit 114a of the transmitter, when explicit
feedback is performed.

[0132] In a case where the wireless communication apparatus is operated as
the terminal of the data transmission in the closed loop type MIMO
communication, and the beamforming is performed to initiate the
transmission of the data packet or the transmission weight matrix for
beamforming is desired to be updated, the sounding packet for exciting
the channel matrix is transmitted to the beamformee to request the
feedback of the CSI information.

[0133] The Mmax×N or M×N channel matrix is constructed
from the CSI information. In either case, because the dimension number of
the channel matrix, which is suppressed to Mmax=N, is subjected to
the singular value decomposition to computecompute transmission weight
matrix V, the circuit size of the beamforming transmission weight matrix
computation unit can be reduced to an order of (Mmax/M)2,
compared with the case where the transmission weight matrix is computed
from the M×N channel matrix.

[0134] When the CSI dimension information is notified to the beamformee,
and the Mmax×N channel matrix is received as the CSI
information, the circuit size of a buffer part for receiving the CSI
information can be reduced to an order of (Mmax/M)2, compared
with a case where an M×N channel matrix is fed back. Because the
CSI information fed back in the channel is reduced from M×N to
Mmax×N, overhead is reduced. Accordingly, the overall
throughput of the system can be improved.

[0135] Due to the reduction in circuit size, it is possible to reduce
power consumption of the apparatus.

[0136] Due to the reduction in circuit size and the reduction of the
overhead of the channel, it is possible to reduce delay related to a
communication process and to reduce time consumed for applying the
beamforming. Thus, it is possible to perform the beamforming based on
fresh channel information. It is possible to minimize the deterioration
of characteristics by the beamforming according to the fresh channel
information.

[0137]FIG. 10 is a flowchart illustrating a process when the wireless
communication apparatus shown in FIGS. 2 and 3 are operated as the
initiator, that is, the beamformer, on the basis of the explicit feedback
procedure. Here, it is assumed that the beamformer includes N antennas
and the beamformee includes M antennas.

[0138] First, the CSI dimension information for describing the maximum
spatial dimension number at the time of the transmission weight matrix
for beamforming is notified to the receiver, which is operated as the
beamformee (step S1). Subsequently, the sounding packet for exciting N
channels is transmitted to request the CSI information (step S2).

[0139] In order to notify the CSI dimension information, it is possible to
employ a method for describing the CSI dimension information in the HT
capability element defined in the EWC specification to be included in a
predetermined management frame. It is also possible to employ a method
for describing the CSI dimension information in the HTC field of the MAC
frame of the sounding packet for requesting the CSI information.
According to the former, the CSI information is notified at the time of
the beacon transmission or the network association. According to the
latter, step S1 and step S2 are simultaneously performed. The CSI
dimension information may not be notified to the beamformee by omitting
step S1.

[0140] Because the channel of N spatial dimensions is excited in the
training signal part of the sounding packet and the beamformee receives
the sounding packet through M antennas, it is possible to estimate the
M×N channel matrix. According to the request for the CSI
information, the CSI information is prepared on the basis of the
estimation channel matrix and a packet having the CSI information in the
data part is returned to the beamformer.

[0141] When the CSI information is received, the beamformer constructs the
channel matrix (step S3), and obtains the transmission weight matrix for
beamforming at the time of the forward data transmission (step S4).

[0142] The Mmax×N or M×N channel matrix is constructed
from the CSI information. In either case, because the channel matrix, in
which the dimension number is suppressed to Mmax×N, is
subjected to the singular value decomposition to compute transmission
weight matrix V, the circuit size of the beamforming transmission weight
matrix computation unit can be reduced to an order of
(Mmax/M)2, compared with the case where the transmission weight
matrix is computed from the M×N channel matrix. Because the CSI
information fed back in the channel is reduced from M×N to
Mmax×N, overhead is reduced. Accordingly, the overall
throughput of the system can be improved.

[0143] The beamforming is performed in a transmission vector having the
transmission signals from the antennas as the element using the
transmission weight matrix for beamforming, and the data packet is
transmitted to the receiver (step S5). It is possible to make an ideal
spatial orthogonal channel by weighting the transmission antennas on the
basis of the channel matrix and performing adequate beamforming directed
to the receiver.

[0144] Due to the reduction in circuit size and the reduction in channel
overhead, the beamformer can reduce delay related to a communication
process and time for applying the beamforming, thereby performing
beamforming based on fresh channel information. It is possible to minimum
the deterioration of characteristics by the beamforming according to the
fresh channel information.

[0145]FIG. 11 a flowchart illustrating a process when the wireless
communication apparatus, shown in FIGS. 2 and 3, is operated as the
receiver, that is, the beamformee, on the basis of the explicit feedback
procedure. Here, it is assumed that the beamformer includes N antennas,
and the beamformee includes M antennas.

[0146] First, the initiator operating as the beamformer receives the CSI
dimension information (step S11). Subsequently, when the sounding packet
is transmitted from the beamformer, the channel of N spatial dimensions
is excited in the training signal part. The beamformee receives the
sounding packet through the M antennas (step S12), and estimates the
M×N channel matrix (step S13). The CSI information is prepared from
the estimation channel matrix, and the packet including the CSI
information in the data part is returned to the beamformer (step S14).

[0147] In step S11, the CSI dimension information is notified using the HT
capability element defined in the EWC specification, or the field of the
MAC frame of the sounding packet. Here, notification of the CSI dimension
information maybe omitted. When the CSI dimension information is
notified, the CSI information is prepared using the channel matrix, in
which the dimension number is suppressed to Mmax×N. When the
CSI dimension information is not notified, the CSI information is
prepared from the estimated M×N channel matrix without any changes.

[0148] The initiator obtains the transmission weight matrix for
beamforming at the time of the forward data transmission using the
channel matrix obtained from the CSI information. The beamforming is
performed in the transmission vector having the transmission signals from
the N antennas as the element using the transmission weight matrix for
beamforming, and the data packet is transmitted.

[0149] The wireless communication apparatus, which is operated as the
beamformee, multiplies the reception vector of the M antennas for
receiving the data packet from the initiator by the reception weight
matrix to perform spatial decoding of the spatial multiplexing signal,
thereby obtaining the signal sequence, which are independent in each
stream. By beamforming, communication can be performed at a high
transmission rate even if the wireless communication apparatus is located
at a place where the packet was difficult to be received in the past.

INDUSTRIAL APPLICABILITY

[0150] Although the invention will be described in detail with reference
to specific embodiments, it is apparent to those skilled in the art that
these embodiments may be modified or substituted without departing from
the scope of the invention as claimed.

[0151] Although the MIMO communication system according to the EWC
specification in IEEE 802.11n is described in the present specification,
the scope of the invention is not limited thereto. As described above,
the MIMO communication system transmits the spatially multiplexed streams
from a first terminal including N antennas to a second terminal including
M antennas. It is appreciated that the invention is applicable to various
other types of communication systems, in which the beamformer performs
the beamforming using the channel information fed back from the
beamformee.

[0152] For simplicity, an embodiment in which the transmission terminal
performs the "direct mapping" for directly mapping the streams to the
antenna branches is described in the present specification. It is
appreciated that the invention is also applicable to employing "spatial
expansion" or a conversion method in which the streams do not one-to-one
correspond to the antenna branches.

[0153] Although an embodiment is described based on IEEE 802.11n standard,
which is extended from IEEE 802.11 standard, the invention is not limited
thereto. The invention is applicable to a variety of wireless
communication systems using an MIMO communication method, such as a
mobile WiMax (Worldwide Interoperability for Microwave) based on IEEE
802.16e, IEEE 802.20 which is a high-speed wireless communication
standard for a mobile object, IEEE 802.15.3c which is a high-speed
wireless PAN (Personal Area Network) using 60 GHz (milliwave) band, a
wireless HD (High Definition) which transmitting an uncompressed HD image
using wireless transmission of 60 GHz (milliwave) band, and a fourth
generation (4G) mobile telephone.

[0154] It should be understood by those skilled in the art that various
modifications, combinations, sub-combinations, and alterations may occur
depending on design requirements and other factors. All such
modifications, combinations, sub-combinations, and alterations are
considered within the scope of the appended claims, or the equivalents
thereof.

Patent applications by Tomoya Yamaura, Tokyo JP

Patent applications in class Having a plurality of contiguous regions served by respective fixed stations

Patent applications in all subclasses Having a plurality of contiguous regions served by respective fixed stations